A. Field of the Invention
The present invention relates to a device and method for correcting the baseline wandering of transmitting signals, and in particular, a device and method for correcting the baseline wandering of transmitting signals by employing a compensation current source to compensate the baseline wandering.
B. Description of the Related Art
FIG. 1 is an equivalent circuit diagram of a conventional receiver 10 comprising resistors 11,12, and current sources 13, 14. The output voltage Vo of the receiver 10 is denoted by Vo=Vopxe2x88x92Von, that is,                                                         Vo              =                              xe2x80x83                            ⁢                              (                                  Vdd                  -                                      (                                          -                      IoR                                        )                                    -                                      (                                          Vdd                      -                      IoR                                        )                                                  )                                                                                        =                              xe2x80x83                            ⁢                              2                ⁢                IoR                                                                        (        1        )            
wherein, Vo is the output voltage of the receiver 10, and R is equivalent output impedance, and Io is the current of the current source received by transformer, and assuming that the effect of the wires or cables can be negligible or is equalized.
In addition, due to the fact that the receiver is output by the transformer, and therefore, the output voltage Vo(S) after Laplace transformation is represented as follows:                               Vo          ⁡                      (            S            )                          =                              Vs            ⁡                          (              S              )                                *                      SL                                          2                ⁢                SL                            +              Ro                                                          (        2        )            
wherein, Vs(S) is the signal source and the signals are digital signal, and SL/(2SL+Ro) is a transfer function of the transformer. Thus, by combining the above Equations (1) and (2), the current Io(S) is obtained and is represented as follows:                               Io          ⁡                      (            S            )                          =                              Vs            ⁡                          (              S              )                                *                      SL                                          (                                                      2                    ⁢                    SL                                    +                  Ro                                )                            ⁢              2              ⁢              R                                                          (        3        )            
From Equation (3),   SL            (                        2          ⁢          SL                +        Ro            )        ⁢    2    ⁢    R  
is a high-pass transfer function. Thus, the DC signal of the Vs(S) will be isolated and cannot pass through, i.e., when the transmitting signals appear a same signal string, xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d, the same signal string is regarded as DC signals, which cause the wandering of the DC level of Vo(S) or is known as baseline wandering. This baseline wandering will cause the receiver to output error signals. FIG. 2 shows the waveforms of the input voltage and the output voltage passed through the transformer. When the voltage level at the input terminal is not uniform, for instance the signals for high potential are longer than that of the low potential, (as shown in FIG. 2(A)). The downward wandering of DC Voltage potential, as shown in 2(B), will form, and this wandering will be accumulated. When the DC voltage potential is wandering downward or upward to a certain extent, i.e., when it is lower or higher than the threshold potential, it will cause error in the output signals of the receiver.
In view of the above drawbacks, it is an object of the present invention to provide a device and method for correcting the baseline wandering of transmitting signals by employing a compensation current source.
In accordance with a device of the invention there is disclosed a device for correcting the transmitting signals baseline wandering at the first output terminal and the second output terminal of the receiver as a result of induction effect of the transformer. The device comprises a compensation current source including a first compensation output terminal and a second compensation output terminal which are respectively connected to the first output terminal and the second output terminal of the receiver. The device further comprises a voltage signal generator for generating control voltage to control the compensation current value of the compensation current source. The voltage signal generator generates the control voltage according to the voltage difference of the first output terminal and the second output terminal of the receiver and a reference voltage. Thereby, the baseline wandering can be corrected by the compensation current value of the compensation current source.
In accordance with a device of the invention there is disclosed a method for correcting the transmitting signals baseline wandering at the first output terminal Vop and the second output terminal Von of the receiver as a result of induction effect of the transformer. The method comprises the steps of:
(a) comparing the voltage difference Vopxe2x88x92Von of the first output terminal and the second output terminal with a voltage threshold Vth, if the voltage difference Vopxe2x88x92Von is higher than the voltage threshold Vth, output a control voltage which is charged by a condenser(capacitor);
(b) comparing the voltage difference Vonxe2x88x92Vop of the second output terminal and the first output terminal with the voltage threshold Vth, if the voltage difference is higher than the voltage threshold Vth, output a control voltage which is reverse-charged by the condenser;
(c) generating a compensation current by the control voltage; and
(d) supplying the compensation current to the first output terminal and the second output terminal.
In accordance with the invention, because a voltage charged by a condenser is used to control the current value of the compensation current, which is supplied to the output terminal, it is effectively in compensating baseline wandering formed as a result of the characteristics of the induction effect of the transformer.
Yet further objections and advantages of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.